Artificial hip joints have long been used to repair damaged hip joints and to restore maximum function to the hip joint. The hip bone includes an acetabulum in the form of a cup-shaped recess within the hip bone. The acetabulum receives a spherical head joined to an upper end of the femur through a neck. The head is rotatably supported within the acetabulum when the natural hip joint is functioning properly.
In hip joint replacement surgery the acetabulum is appropriately modified to receive an acetabular cup therein. The acetabular cup is typically a rigid hemispherical structure formed of titanium, cobalt chrome and polyethylene or other biocompatible material. The cup has an exterior surface which is roughened such as with a porous coating or grit blasted to encourage bone interdigitation. A top bore is provided passing through an uppermost portion of the acetabular cup. This top bore is utilized in properly aligning the acetabular cup and to optionally receive a screw plug to prevent fluid migration therethrough. Side holes are typically provided passing through the acetabular cup should additional bone screws or other fasteners be needed to secure the acetabular cup to the hip bone.
The interior surface of the acetabular cup has this top bore and side holes passing therethrough, and so is not optimally formed to bear loads associated with the head of the artificial hip joint directly there against. Hence, a liner is provided which mates within the acetabular cup with an outside surface of the liner abutting the interior surface of the acetabular cup. The inside surface of the liner is hemispherical and provides the bearing surface to support the ball of an artificial hip joint which is secured to an upper end of the femur and rotatably supports the head of the femur therein. The liner is secured to the acetabular cup through a locking ring which fits within both a groove in the interior surface of the acetabular cup and within a seat in the outside surface of the liner.
One particular problem with prior art acetabular cups and liners of artificial hip joints is that the top bore, unless plugged with an optional screw plug, provides a region for the passage of fluids from adjacent the hip bone through the acetabular cup and into the artificial hip joint, or for the passage of wear debris from within the artificial hip joint into contact with the hip bone. Wear debris can discourage bone growth into the exterior surface of the acetabular cup, increasing the possibility of failure of the artificial hip joint. Fluid migrating through the top bore into the artificial hip joint can decrease the operational effectiveness of the artificial hip joint. Hence, it is desirable that this top bore be sealed.
Some prior art acetabular cups and liners are known to include posts or other structures passing into the top bore of the acetabular cup. However, such posts have not been taught to seal such a passage in the acetabular cup, but rather have been provided to secure the liner within the acetabular cup or to provide for alignment of the liner within the acetabular cup. Representative prior art devices are disclosed by Kenna (U.S. Pat. No. 4,666,450), Willi (U.S. Pat. No. 5,549,696) and Forte (U.S. Pat. No. 4,695,282). Accordingly, a need exists for a liner which, in addition to providing the proper bearing surface for the ball of the femur, additionally seals a top bore passing through the acetabular cup.
Additionally, locking rings are known in the art for securing a liner within an acetabular cup. For instance, the patent to Rohr (U.S. Pat. No. 5,383,938), Thongpreda (U.S. Pat. No. 5,314,491) and Ramos (U.S. Pat. No. 4,380,090) each teach generally circular locking rings with a break therein and with two free ends. The free ends can be drawn together or pushed apart to insert such locking rings into associated grooves and seats within the acetabular cup and liner to secure the liner within the acetabular cup. Such locking rings, while generally effective, can be difficult to insert, and typically once the liner has been secured within the acetabular cup, such locking rings are difficult, if not impossible, to remove, should it be desired by a surgeon to remove the liner after it has been placed within the acetabular cup with the locking ring. Accordingly, a need exists for a locking ring which is easy to insert to secure a liner within an acetabular cup and which can be easily removed to allow the liner to be removed from the acetabular cup if desired, particularly by a surgeon during a surgical procedure with the minimum potential complication.